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Handout9-ComplexEquilibria

# Handout9-ComplexEquilibria - CEM 834 COMPLEXATION...

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COMPLEXATION EQUILIBRIA FALL 2005 Much of the formalism derived for acids and bases may be easily extended to systems where complexes are formed by metal ions with neutral or ionic groups called ligands. Acid–base systems may be considered as a special case in which complexes are formed by combination of hydrogen ions with a central group such as PO 4 3 - . Metal ions in solution are never completely uncomplexed, since there are always a number of tightly bound solvent molecules attached to the metal. Thus Al 3+ in aqueous solution is usually present as Al(H O) 2 6 3 + . Because there is always excess water in aqueous solution, we shall usually leave the water ligands out of the formulae. Complexation equilibria are often more difficult to treat than polyprotic acid–base equilibria because there are usually more species present. However, once the ligand concentration is known in a complex system, the calculation of the remaining concentrations is straightforward, just as it is for acid–base systems once the pH is known. Several of the graphical representations used in acid–base systems are directly applicable to complex ion systems. In these representations, the ligand concentration pL is the master variable instead of pH. I. Complexation without Secondary Equilibria A. Simultaneous Equation Approach We shall consider first the calculation of concentrations of various species of complexes when excess ligand is present. 1. Calculate the concentration of all species in a solution of 1.00 M HCl and 0.010 M Cd(NO ) 3 2 , neglecting activity corrections and hydrolysis of Cd 2+ . The equilibria and formation constants are 2 f c1 + f 2 c2 f 2 3 c3 2 f 3 4 c4 Cd Cl CdCl K 21 CdCl Cl CdCl K 7.9 CdCl Cl CdCl K 1.23 CdCl Cl CdCl K 0.35 The mass balance equations for Cl and Cd 2+ are [ ] [ ] [ ] [ ] [ ] M 00 . 1 CdCl 4 CdCl 3 CdCl 2 CdCl Cl C 2 4 3 2 Cl = + + + + = - - + - - (1) [ ] [ ] [ ] [ ] [ ] M 010 . 0 CdCl

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Handout9-ComplexEquilibria - CEM 834 COMPLEXATION...

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